The primary reason such items are frowned upon is due to redundancy, lack-thereof. Using such a cable means all of your server's power-inputs are being fed by the same circuit so when that circuit dies (or the PDU it's connected to, I've had that happen) so does the server. Colos strongly recommend Primary and Secondary circuits for just this reason and want to see multi-PSU servers plugged into two circuits.
Way back in the day I had a group of machines that shipped with a single 3-way Y cable and 3 normal power cables for a large (7U if I remember right) 3 PSU system. The data-center I was working in at the time (this was about 1999) didn't have enough power-outlets for that kind of thing, so we ended up using the Y cable; 2 legs of the Y on one UPS, and a straight up power-cable for the 3rd PSU to the second UPS. 3-PSU systems are thankfully much less common now.
PSU Load-balancing, or is it switching?
There are differences to how power-supplies handle loading. As various power-supply benchmarks have shown, peak efficiencies are reached once you get over 50% loading. There are gains to be had for running things all on one PSU, it's more likely to be efficient. It is for this reason that some server manufacturers draw all of a server's current through a single PSU and switch to the other one when a failure happens or a whim strikes; a 230 Watt system will get best efficiency from its dual 400 Watt PSUs by running all the load through only one PSU.
Such switching systems only draw from one PSU, and therefore one circuit if using fully separated power circuits, at a time.
The downside to switching systems is that load can move unpredictably in the community of PSUs connected to a certain circuit. If enough of them throw their weight to a single circuit it can overload it. This is bad power-design, since you want to design things so that you can lose a full circuit and have things stay up, but it's still something that trips up systems engineers.
Load-balancing servers draw equal amounts of current from both PSUs. This gives predictability in circuit loading, though can still cause blown breakers if the Systems Engineers load their circuits over 50% and a circuit dies forcing the PSUs to draw 100% from one circuit, which now exceeds its rating. Again, bad power design but it's a common mistake.
Startup loads
There are two kinds of startup-loading:
- Everything runs flat out until BIOS (or OS Boot, or app-load) catches up and things calm down.
- Inrush current loading right as things get turned on.
The first is something we're all familiar with. That 120 disk SAS array may draw only 4000 Watts when running normally, but if all the disk-shelves restart at the same time it may draw 6500 Watts.
The same holds true for servers. Fans run at full speed, yes. CPUs run at full speed for a bit, yes. RAM is run at full voltage during post, yes. It's likely to draw as much as it can draw during those first stages of POST but rapidly drops off as the BIOS hands things off to the OS and power regimes take over. A server that normally draws 110 Watts during normal usage may temporarily draw as much as 200W for those first few phases.
It's this temp loading that most people think of when they say things like, "it runs the power-supplies at full on startup". Those 400 Watt power-supplies plugged into a server that draws 230W on a busy day aren't going to draw 400 Watts, they'll draw 230W... combined.
The second isn't well known, but when people run across it they get worried. This is inrush current, and takes a few milliseconds during which draw can be quite a lot higher than it normally is. The inrush current for IT devices with AC to DC converters in them (which is all of them) almost always happens twice:
- One time when the cable is plugged in, as the pre-power stage gets power. It's this stage that allows the power-button on the front to power on the device.
- A second time when the main distribution stage powers up and starts the device.
Because of the timings, this only becomes a factor when restoring power to a dead circuit. All those devices powering on at exactly the same time can do weird things to the power on that circuit, and that can cause damage all by itself. Doing a staged startup alleviates this.
This is the other area people think of when they say things like "power-supplies run full-tilt on startup", since each PSU has its own inrush current. But as I said, this lasts for a few milliseconds and comes in two stages.
